It is known to use a bit-to-bit polarization interleaved bitstream to reduce e.g. non-linear effects during transmission. It is also known to compensate the so-called polarization mode dispersion with the help of birefringent elements. However, this compensation requires an eye monitor which does not cope well with the mentioned bit-to-bit polarization interleaved bitstream. The same is valid for the decision circuit of known optical transmission systems.